Nadal is bidding to win the elite year-end event for the first time. He will face either Novak Djokovic or Stanislas Wawrinka in Monday’s final.

“It is the perfect way to finish the season, and it is probably one of my best seasons,” said Nadal, who reclaimed the No. 1 spot this year after coming back from a knee injury in February. “In a tournament where I have lost before and have not had the most luck in, it is great.”

Federer, who has been hampered by a back injury this season, has won only one title over the past 14 months. He played aggressively early but faded after losing the first set, making too many mistakes to threaten the Spaniard.

Nadal and Federer treated the O2 Arena fans to some superb exchanges early on, reminiscent of the classic matches that have punctuated their nine-year rivalry. But Nadal was much sharper at the important moments, converting all four of his break points, while Federer sometimes looked like a shadow of his old self, making 32 unforced errors and hitting some poor volleys.

“I struggled to stay consistent throughout the match,” Federer said.

Federer, who beat Juan Martin del Potro on Saturday to qualify for the semis while Nadal had a rest day, also looked tired toward the end.

Federer missed an early chance when he failed to convert three break points in the sixth game. Nadal took advantage of his first opportunity to break to go ahead 5-4 with a superb forehand winner.

Federer sent the crowd into rapturous cheers as he broke back immediately after winning a long rally with a forehand down the line.

Federer could not build on the momentum and dropped his serve in the next game, before Nadal closed out the set. Federer then struggled with his rhythm and was broken twice in the second set, with Nadal sealing the victory when Federer sent a volley long.

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